1. Field of the Invention
This invention relates to a modified crystalline aluminosilicate zeolite and a process of making the same.
2. Prior Art
There are known a variety of zeolites, of which the so-called ultrastable Type Y zeolites have become widely acclaimed for their commerical value.
A typical example of the process for preparing such zeolite compositions is disclosed in U.S. Pat. No. 3,929,672 which comprises calcining an ammonium-sodium Y zeolite containing about 0.6-5 weight-percent of sodium as Na.sub.2 O, said calcining being carried out at a temperature between about 600.degree. and 1650.degree. F. in contact with at least about 0.2 psi of water vapor for a sufficient time to substantially reduce the unit cell size of said zeolite and bring it to a value between about 24.40 and 24.64 .ANG.; and subjecting the calcined zeolite to further ammonium ion exchange under conditions adjusted to replace at least about 25 percent of its residual zeolite sodium ions with ammonium ions and produce a final product containing less than about 1 weight-percent Na.sub.2 O.
Another prior art process is taught in U.S. Pat. No. 3,506,400 whose claim 1 reads on a process for increasing the silica/alumina mole ratio in the crystal lattice of a crystalline aluminosilicate zeolite of the molecular sieve type, which molecular sieve is at least partially in its hydrogen form, said process comprising in combination: treating said molecular sieve at an elevated temperature within the range of about 800.degree. to 1500.degree. F. in an atmosphere containing at least 2% water for a period of time sufficient to effect removal of at least some alumina tetrahedra from said molecular sieve crystal lattice, said alumina forming an amorphous phase in said molecular sieve; and then contacting the thus treated molecular sieve with an agent, selected from the group consisting of the dilute mineral acids and the organic acid chelating agents, selective for the removal of alumina, whereby an aluminosilicate zeolite molecular sieve is obtained having an enhanced silica/alumina mole ratio with at least substantial retention of the degree of crystallinity of said molecular sieve.
Because of the possibility of acid treatment of zeolites such as of faujasite type leading to the disruption of the crystalline structure of the zeolite, both of the above cited prior art processes rely on partial exchange of alkali metal in the zeolite with ammonium ions to somewhat enhance its hydrothermal stability and on steam calcination to reduce the unit cell size of the zeolite thereby providing increased stability to acids and heat, followed by dealumination to provide greater SiO.sub.2 /Al.sub.2 O.sub.3 ratios. It was further necessary after calcination to again subject the zeolite to exchange with ammonium ions so as to achieve sufficient reduction of its alkali metal ions. These steps of treatment add up to time-consuming and costly manufacture of the intended ultrastable zeolites. The prior processes have a further drawback in that the resultant zeolites have somewhat defective crystal structure and insufficient thermal stability due to the lack of positive silicon supply to the sites of the structure which have been dealuminated.
Improved processes have been proposed for the manufacture of modified zeolites rendered relatively free of structural defects or degradation by replacing aluminum ions with silicon ions. One such process is disclosed in EP-0,072,397 in which the zeolite is subjected to gas-phase reaction with gaseous halogenated silane. This compound is highly reactive with water and easily polymerizable per se and therefore difficult to handle. Another such process is shown in U.S. Pat. No. 4,711,770 in which the zeolite is treated in an aqueous phase with fluorosilicate. Still another such process is taught in EP-0,244,056 which employs for example ammonium hydrogen fluoride (NH.sub.4.HF) as a fluoride ion source. These two processes commonly use fluorine compounds which tend to produce hydrogen fluoride in the reaction system. The presence of hydrogen fluoride poses a corrosion problem which demands installation of corrosion-proof manufacturing equipment and a further problem in that a starting zeolite if containing an alkali metal would suffer separation from cryolite (Na.sub.3 AlF.sub.6).